Thorium and uranium trace ICP-MS analysis for AMoRE project.

AMoRE (Advanced Mo-based Rare process Experiment) is an international collaboration searching for the neutrinoless double-beta decay of the Mo isotope with cryogenic detectors using molybdate (MoO)-based scintillation crystals. The process requires that the detector apparatus and its components, including bolometric crystals and thus initial materials used for the crystal growth, be extremely low in radioactive isotopes having decays that may generate background noise signals in the region of interest. The present study summarizes an ICP-MS assay program conducted for the AMoRE experiment. Firstly, the MoO powder, the main component of the crystals, was studied in the analysis. Before crystal synthesis, enriched MoO powder was purified at the Center for Underground Physics (CUP). To ensure its radio purity, a sample preparation technique with a UTEVA® resin was developed for Th and U analysis with ICP-MS. The recovery yield was over 90% for the extraction procedure, and the detection limits for Th and U were 2.3 and 1.0 ppt, respectively. To determine the most appropriate material for the detector frame and shielding, several types of high-purity Cu were measured: Cu-OFE (Aurubis and Mitsubishi Materials) and Cu-NOSV (Aurubis). Similarly, a solid-phase extraction was applied for Th and U analysis, and detection limits were calculated at 0.1 and 0.2 ppt, respectively. The 3M Vikuiti™ ESR film, the closest part to the crystal in the detector assembly, was used as a light reflector. Two types of Vikuiti film, a roll and a sheet, were checked for radiopurity via full decomposition using a microwave ashing system. The procedural Detection Limits were achieved at a level of about 1 ppt.